Omeruo, 25, starred with Leganes on loan last term, making 31 appearances for the club, 30 of which were starts.
Earlier this month, Goal reported that Chelsea were holding out for £6 million (€7m/$8m) for Omeruo, but the Blues have accepted a bid of around £4.6m.
Omeruo signed for Chelsea in 2012 from Standard Liege, but was never able to make the breakthrough into the first team at Stamford Bridge.
The Nigeria international took in several loan stints during his time at Chelsea, including with ADO Den Haag in the Netherlands and Middlesbrough in England, as well as Kasımpasa and Alanyaspor in Turkey.
But Omeruo found a home at Leganes in his final loan spell with the Blues, and parlayed his strong performances last term into a permanent move.
Earlier this summer, Omeruo spoke of his desire to leave Chelsea to further his career.
"I think now’s the time for me to leave, to find a place, to be my own man," he said. "I’m married with a baby now, so I need stability.
"Chelsea have done amazingly for me, and for my family as well, but it’s time for me to reach my potential and get where I think I deserve to be."
Though Chelsea have been unable to bring in any players this summer due to a two-window transfer ban, there could still be more players leaving the club.
Tiemoue Bakayoko, Kenedy and Davide Zappacosta are all candidates to leave the club before the European transfer window slams shut in early September.
Due to the club's transfer ban, Christian Pulisic is the only new arrival at Stamford Bridge this season, with the American signed in January and loaned back to Borussia Dortmund for the remainder of the 2018-19 season.